New pyridine carboxamide ligands and their complexation to copper(II). X-ray crystal structures of mono-, di, tri- and tetranuclear copper complexes

Article abstract of DOI:10.1039/b316519a

Seven new pyridine dicarboxamide ligands H₂L^1-7 have been synthesised from condensation reactions involving pyridine-2,6-dicarboxylic acid (H₂dipic), pyridine-2,6-dicarbonyl dichloride or 2,6-diaminopyridine with heterocyclic amine or carboxylic acid precursors. Crystallographic analyses of N,N′-bis(pyridyl)pyridine-2,6-dicarboxamide monohydrate (H₂L⁸·H₂O), N,N′-bis[2-(2-pyridyl)methyl]pyridine-2,6-dicarboxamide and N,N′-bis[2-(2-pyridyl)ethyl]pyridine-2,6-dicarboxamide monohydrate revealed extensive intramolecular hydrogen bonding interactions. 2,6-Bis(pyrazine-2-carboxamido)pyridine (H₂L⁶) and 2,6-bis(pyridine-2-carboxamido)pyridine (H₂L⁷) reacted with copper(II) acetate monohydrate to give tricopper(II) complexes [Cu ₃(L)₂(μ₂-OAc)₂]. X-Ray crystallography confirmed deprotonation of the amidic nitrogen atoms and that the (L^6,7)^2- ligands and acetate anions hold three copper(II) ions in approximately linear fashion. H₂L⁸. Reacted with copper(II) tetrakis(pyridine) perchlorate to give [Cu(L ⁸)(OH₂)]₂· 2H₂O, in which (L⁸)^2- was tridentate through the nitrogen atoms of the central pyridine ring and the deprotonated carboxamide groups at one copper centre, with one of the terminal pyridyl rings coordinating to the other copper atom in the dimer. The corresponding reaction using H₂L⁷ gave [Cu₃(L⁷)₂(py)₂][ClO ₄]₂, which transformed during an attempted recrystallisation from ethanol under aerobic conditions to a tetracopper(II) complex [Cu₄(L⁷)₂(L⁷-O) ₂].

Full text of DOI:10.1039/b316519a

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New pyridine carboxamide ligands and their complexation to
copper(II). X-Ray crystal structures of mono-, di, tri- and
tetranuclear copper complexes
Sneh L. Jain, Pravat Bhattacharyya,* Heather L. Milton, Alexandra M. Z. Slawin,
Joe A. Crayston and J. Derek Woollins*
School of Chemistry, University of St Andrews, St Andrews, Fife, Scotland, UK KY16 9ST.
E-mail: jdw3@st-and.ac.uk; Fax: (ϩ44)-1334-463808; Tel: (ϩ44)-1334-463861
Received 17th December 2003, Accepted 11th February 2004
First published as an Advance Article on the web 23rd February 2004
Seven new pyridine dicarboxamide ligands H2L1–7 have been synthesised from condensation reactions involving
pyridine-2,6-dicarboxylic acid (H2dipic), pyridine-2,6-dicarbonyl dichloride or 2,6-diaminopyridine with heterocyclic
amine or carboxylic acid precursors. Crystallographic analyses of N,NЈ-bis(2-pyridyl)pyridine-2,6-dicarboxamide
monohydrate (H2L8ؒH2O), N,NЈ-bis[2-(2-pyridyl)methyl]pyridine-2,6-dicarboxamide and N,NЈ-bis[2-(2-pyridyl)-
ethyl]pyridine-2,6-dicarboxamide monohydrate revealed extensive intramolecular hydrogen bonding interactions.
2,6-Bis(pyrazine-2-carboxamido)pyridine (H2L6) and 2,6-bis(pyridine-2-carboxamido)pyridine (H2L7) reacted with
copper() acetate monohydrate to give tricopper() complexes [Cu3(L)2(µ2-OAc)2]. X-Ray crystallography confirmed
deprotonation of the amidic nitrogen atoms and that the (L6,7 2Ϫ ligands and acetate anions hold three copper() ions
)
in approximately linear fashion. H2L8. Reacted with copper() tetrakis(pyridine) perchlorate to give [Cu(L8)(OH2)]2ؒ
2H2O, in which (L8)2Ϫ was tridentate through the nitrogen atoms of the central pyridine ring and the deprotonated
carboxamide groups at one copper centre, with one of the terminal pyridyl rings coordinating to the other copper
atom in the dimer. The corresponding reaction using H2L7 gave [Cu3(L7)2(py)2][ClO4]2, which transformed during
an attempted recrystallisation from ethanol under aerobic conditions to a tetracopper() complex [Cu4(L7)2(L7–O)2].
Introduction
Experimental
The carboxamide [–C(O)NH–] group, ubiquitous throughout
Nature in the primary structure of proteins, is an important
ligand construction unit for coordination chemists. Pyridine
carboxamides, a burgeoning class of multidentate ligands con-
taining this linkage, are available from condensation reactions
between pyridyl-bearing amine or carboxylic acid precursors,
promoted by coupling agents such as 1,1Ј-carbonyldiimadazole,
diphenoxyphosphoryl azide or triphenylphosphite.1–5 Upon
deprotonation of the carboxamide nitrogen atom, this centre
and the pyridyl ring(s) of the anion chelate to metal ions.
Pyridine carboxamide ligands have found use in asymmetric
catalysis,2,6 molecular receptors,7,8 dendrimer synthesis9 and
platinum() complexes with antitumour properties.4
The behaviour of pyridine carboxamides towards bio-
logically relevant d-block metals has been widely investigated,
with copper being a particularly fruitful element for study.
Copper() is integral to the active site of numerous electron
transfer and oxygenase metalloproteins, moreover these biosites
often contain several copper ions in very close proximity to one
another, e.g. ascorbate oxidase and laccase have four copper
atoms in the active site. Pyridine dicarboxamide ligands
support a range of coordination numbers, geometries and
nuclearities for copper().3,10–13
Apart from the general interest mentioned above there
has recently been a report14 on the use of related pyrazine
carboxamide ligands in the control of molecular architectures.
Here we describe the coordination chemistry of copper() with
new polydentate pyridine dicarboxamide ligands prepared
from pyridine-2,6-dicarboxylic acid (H2dipic), pyridine-2,6-
dicarbonyl dichloride or 2,6-diaminopyridine. X-Ray crystal-
lography revealed that mononuclear through to tetranuclear
complexes are supported by these ligands, dictated by the
necessity to avoid the formation of four membered chelate rings
and the flexibility of the ligand backbone.
All commercial materials and solvents were used as supplied,
pyridine and triethylamine were dried and distilled from barium
oxide and calcium hydride respectively. Ligand syntheses and
complexation reactions were conducted under dinitrogen or
argon unless otherwise stated, subsequent work-up and re-
crystallisation procedures were performed under aerobic con-
ditions. H2L8 was prepared by a triphenylphosphite promoted
condensation reaction between 2-aminopyridine and H2dipic,
H2L9 and H2L10 were prepared from pyridine-2,6-dicarbonyl
dichloride and 2-(aminomethyl)pyridine or 2-(aminoethyl)-
pyridine respectively in the presence of triethylamine.12,15 H2L8ؒ
H2O, H2L9 and H2L10ؒH2O were crystallised from chloroform–
diethyl ether. H2L11 was prepared from Hpic and 1,2-
phenylenediamine.16,17
1H NMR (270.0 MHz) and IR spectra (pressed KBr discs)
were recorded on a JEOL DELTA GSX 270 NMR and Perkin-
Elmer System 2000 FTIR/Raman spectrometers respectively.
Microanalyses were carried out on a Carlo-Erba 1108 instru-
ment by the University of St Andrews Microanalytical Service.
EI, ES and FAB MS (positive ionisation mode, 3-nitrobenzyl
alcohol matrix) were recorded by the EPSRC National Mass
Spectrometry Service (Swansea) and at the University of
St Andrews. Magnetic susceptibility measurements were
performed on an MPMS–XL squid magnetometer (Quantum
Design, San Diego). Cyclic voltammetry and chronoampero-
metry for 1 and 2 were performed using a PC-controlled EG &
G PAR 273A potentiostat, with a three-electrode cell compris-
ing a platinum disc working electrode (0.5 mm diameter),
a platinum wire auxiliary electrode and a silver wire quasi-
reference electrode which was standardised against the wave of
added ferrocene.
CAUTION: Although we have encountered no difficulties,
perchlorate salts are potentially explosive and should be
handled with care.
D a l t o n T r a n s . , 2 0 0 4 , 8 6 2 – 8 7 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
862

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Pyridine-2,6-dicarbonyl dichloride
2,6-Bis(pyrazine-2-carboxamido)pyridine H2L6
H2dipic (5.0 g, 30.0 mmol) was added to thionyl chloride
(50 cm3) and heated to reflux under dinitrogen for 6 h. The
reaction mixture was allowed to cool, the solvent was removed
under reduced pressure and the product dried in vacuo. Yield
6.12 g (100%). Selected IR bands (cmϪ1): 1752s (νCO).
Compounds H2L1–3 and H2L6,7 were prepared as described
here for H2L7 on the same scale, illustrated for H2L7.
Under an argon atmosphere, Hpic (4.67 g, 38 mmol) was
added as a solid in one portion to a suspension of 2,6-diamino-
pyridine (3.0 g, 18 mmol) in pyridine (10 cm3) and the mixture
stirred at 40 ЊC for 40 min. Triphenylphosphite (9.5 cm3, 36
mmol) was added dropwise over 10 min, after which time the
temperature of the mixture was increased to 90–100 ЊC and
stirring continued for a further 4 h. On cooling to room
temperature the colourless precipitate was filtered off, washed
with water (20 cm3), methanol (20 cm3) and recrystallised from
chloroform. Yield 4.25 g (74%).
Yield 40%. Found (calc. for C15H11N7O2): C 55.62 (56.07), H
3.41 (3.45), N 30.68 (30.52)%. δH (CDCl3): 10.01 (br s, 2H, NH),
9.48 (d, 2H, J = 1.5, C4H3N2), 8.79 (d, 2H, J = 2.5, C4H3N2),
8.58 (dd, 2H, J = 1.5 and 2.5, C4H3N2), 8.15 (d, 2H, J = 8,
C5H3N), 7.83 (t, 1H, J = 8 Hz, C5H3N). Selected IR bands
(cmϪ1): 3354s (νNH), 1702s (νCO). EI MS: m/z 321, Mϩ.
2,6-Bis(pyridine-2-carboxamido)pyridine H2L7
Yield 74%. Found (calc. for C17H13N5O2): C 64.35 (63.94), H
3.49 (4.10), N 22.07 (21.93)%. δH(CDCl3): 10.43 (br s, 2H, NH),
8.66 (ddd, 2H, J = 1, 2 and 5, C5H4N), 8.31 (dt, 2H, J = 1 and 8,
C5H4N), 8.18 (d, 2H, J = 8, C5H3N), 7.93 (dt, 2H, J = 1.5 and 8,
C5H4N), 7.85 (t, 1H, J = 8, C5H3N), 7.51 (ddd, 2H, J = 1, 5 and
8 Hz, C5H4N). Selected IR bands (cmϪ1): 3340m (νNH), 1701s
(νCO). EI MS: m/z 319, Mϩ.
N,NЈ-Bis(2-pyridyl)pyridine-2,6-dicarboxamide H2L8
Compounds H2L4,5 were prepared by identical procedures,
illustrated for H2L4.
Yield 85%. Found (calc. for C17H11N5O2ؒH2O): C 60.76 (60.88),
H 4.28 (3.91), N 21.07 (20.90)%. δH (CDCl3): 10.59 (br s, 2H,
NH), 8.51 (d, 2H, J = 8, C5H4N), 8.44 (d, 2H, J = 8, C5H3N),
8.35 (dd, 2H, J = 1 and 4, C5H4N), 8.15 (t, 1H, J = 8, C5H3N),
7.77 (dt, 2H, J = 2 and 8, C5H4N), 7.10 (dd, 2H, J = 5 and 8 Hz,
C5H4N). Selected IR bands (cmϪ1): 3293m (νNH), 1698s (νCO). EI
MS: m/z 319, Mϩ.
Pyridine-2,6-dicarbonyl dichloride (5.0 g, 24.5 mmol), 2-
aminopyrimidine (4.66 g, 49.0 mmol) and triethylamine (5.05 g,
50.0 mmol) were refluxed in thf (300 cm3) for 5 h. Triethylamine
hydrochloride was filtered off, the filtrate was evaporated
in vacuo and the residue redissolved in acetonitrile (50 cm3). The
solution was stored at Ϫ20 ЊC overnight to give a colourless
solid. Yield 3.1 g (40%).
N,NЈ-Bis[2-(2-pyridyl)methyl]pyridine-2,6-dicarboxamide H2L9
N,NЈ-Bis(2-thiazolyl)pyridine-2,6-dicarboxamide H2L1
Yield 78%. Found (calc. for C19H17N5O2): C 64.34 (65.69), H
4.69 (4.93), N 19.91 (20.16)%. δH (CDCl3): 8.99 (br t, 2H, J = 5,
NH), 8.46 (d, 2H, J = 5, C5H4N), 8.34 (d, 2H, J = 8, C5H3N),
8.00 (t, 1H, J = 8, C5H3N), 7.64 (dt, 2H, J = 1.5 and 8, C5H4N),
7.33 (d, 2H, J = 8, C5H4N), 7.16 (dd, 2H, J = 5 and 8, C5H4N),
4.76 (d, 4H, J = 6 Hz, CH2). Selected IR bands (cmϪ1): 3306m
(νNH), 1676s (νCO). EI MS: m/z 347, Mϩ.
Yield 77%. Found (calc. for C13H9S2N5O2ؒ2H2O): C 42.82
(42.49), H 1.59 (3.57), N 19.01 (19.06)%. δH (CDCl3): 12.20 (br
s, 2H, NH), 8.51 (d, 2H, J = 8, C5H3N), 8.22 (t, 1H, J = 8,
C5H3N), 7.52 (d, 2H, J = 4, C3H2NS), 7.06 (d, 2H, J = 4 Hz,
C3H2NS). Selected IR bands (cmϪ1): 3250s (νNH), 1682s (νCO).
EI MS: m/z 331, Mϩ.
N,NЈ-Bis[2-(2-pyridyl)ethyl]pyridine-2,6-dicarboxamide H2L10
N,NЈ-Bis[2-(5-chloro)pyridyl]pyridine-2,6-dicarboxamide H2L2
Yield 72%. Found (calc. for C21H21N5O2): C 66.81 (67.18), H
5.54 (5.64), N 18.63 (18.65)%. δH (CDCl3): 8.74 (br t, 2H, J = 6,
NH), 8.48 (d, 2H, J = 5, C5H4N), 8.29 (d, 2H, J = 8, C5H3N),
7.98 (t, 1H, J = 8, C5H3N), 7.60 (dt, 2H, J = 1.5 and 8, C5H4N),
7.20 (d, 2H, J = 8, C5H4N), 7.14 (dd, 2H, J = 5 and 8, C5H4N),
3.91 (q, 4H, J = 6, CH2), 3.14 (t, 4H, J = 6 Hz, CH2). Selected IR
bands (cmϪ1): 3319s (νNH), 1674 (νCO). EI MS: m/z 375, Mϩ.
Yield 88%. Found (calc. for C17H11N5O2Cl2ؒH2O): C 50.47
(50.26), H 2.48 (3.22), N 17.19 (17.24)%. δH (CDCl3): 10.75
(br s, 2H, NH), 8.50 (d, 2H, J = 8, C5H3N), 8.46 (d, 2H, J = 9,
ClC5H3N), 8.35 (d, 2H, J = 3, ClC5H3N), 8.18 (t, 1H, J = 8,
C5H3N), 7.80 (dd, 2H, J = 3 and 9 Hz, ClC5H3N). Selected IR
bands (cmϪ1): 3268m (νNH), 1697s (νCO). EI MS: m/z 387, Mϩ.
N,NЈ-Bis[2-(4-methyl)pyridyl]pyridine-2,6-dicarboxamide H2L3
1,2-Bis(pyridine-2-carboxamido)benzene H2L11
Yield 40%. Found (calc. for C19H17N5O2ؒH2O): C 62.27 (62.44),
H 4.99 (5.24), N 19.33 (19.16)%. δH (CDCl3): 10.96 (br s, 2H,
NH), 8.50 (d, 2H, J = 8, C5H3N), 8.35 (s, 2H, CH3C5H3N), 8.22
(d, 2H, J = 5, CH3C5H3N), 8.15 (t, 1H, J = 8, C5H3N), 6.93 (d,
2H, J = 5 Hz, CH3C5H3N), 2.42 (s, 6H, CH3). Selected IR bands
(cmϪ1): 3299m (νNH), 1702s (νCO). EI MS: m/z 347, Mϩ.
Found (calc. for C18H14N4O2): C 67.91 (67.91), H 4.43 (4.21), N
17.76 (17.60)%. δH (CDCl3): 10.25 (br s, 2H, NH), 8.55 (ddd,
2H, J = 1, 2 and 5, C5H4N), 8.30 (dt, 2H, J = 1 and 8, C5H4N),
7.88 (m, 4H, C5H4N ϩ C6H4), 7.45 (ddd, 2H, J = 1, 5 and 8 Hz,
C5H4N), 7.29 (m, 2H, C6H4). Selected IR bands (cmϪ1): 3316s
(νNH), 1677s (νCO). EI MS: m/z 318, Mϩ.
N,NЈ-Bis(2-pyrimidinyl)pyridine-2,6-dicarboxamide H2L4
3,4-Bis(pyridine-2-carboxamido)toluene H2L12
Yield 40%. Found (calc. for C15H11N7O2): C 55.60 (56.07), H
2.95 (3.45), N 24.74 (30.51)%. δH (CDCl3): 11.07 (s, 2H, NH),
8.74 (d, 4H, J = 5, C4H3N2), 8.60 (d, 2H, J = 8, C5H3N), 8.18
(t, 1H, J = 8, C5H3N), 7.12 (t, 2H, J = 5 Hz, C4H3N2). Selected IR
bands (cmϪ1): 3327m (νNH), 1718s (νCO). EI MS: m/z 321, Mϩ.
Hpic (2.22 g, 0.018 mol) was suspended in dry pyridine (10 cm3)
under argon. 3,4-Diaminotoluene (4.64 g, 0.038 mol) was
added and the mixture was stirred for 40 min at 40 ЊC. Initially
a colourless precipitate appeared, which finally formed an
emulsion. Triphenylphosphite (9.5 cm3, 0.036 mol) was added
dropwise, the temperature of the reaction mixture was
increased to 90–100 ЊC and the mixture stirred for 5 h. After
cooling to room temperature overnight methanol (20 cm3) and
water (20 cm3) were added, the colourless precipitate was
filtered off and recrystallised from chloroform–diethyl ether.
Yield 5.38 g (90%). Crystals suitable for X-ray analysis were
grown from chloroform–diethyl ether at Ϫ20 ЊC. Found (calc.
for C19H16N4O2): C 69.76 (68.66), H 4.53 (4.85), N 17.22
N,NЈ-Bis[2-(2-naphthyl)methyl]pyridine-2,6-dicarboxamide
H2L5
Yield 41%. Found (calc. for C29H23N3O2ؒ0.5H2O): C 76.10
(76.60), H 4.93 (5.32), N 9.22 (9.24)%. δH (CDCl3): 8.33 (m, 2H,
aromatic H), 7.98–7.73 (m, 9H, aromatic H), 7.46–7.24 (m, 9H,
aromatic H), 5.00 (d, 4H, J = 6 Hz, CH2). Selected IR bands
(cmϪ1): 3281m (νNH), 1656s (νCO). EI MS: m/z 445, Mϩ.
D a l t o n T r a n s . , 2 0 0 4 , 8 6 2 – 8 7 1
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Table 1 Details of data collections and structural refinements for H2L8ؒH2O, H2L9, H2L10ؒH2O and H2L12
H2L8ؒH2O
H2L9
H2L10ؒH2O
H2L12
Empirical formula
M
C17H15N5O3
337.34
C19H17N5O2
347.38
C21H23N5O3
393.44
C19H16N4O2
332.36
T /K
λ/Å
Crystal system
Space group
293(2)
293(2)
293(2)
125(2)
0.71073
Orthorhombic
P212121
0.71073
Monoclinic
I2/a
1.54178
Triclinic
¯
P1
0.71073
Monoclinic
C2/c
a/Å
b/Å
c/Å
α/Њ
3.8239(4)
16.5567(17)
24.639(3)
90
90
90
1560.0(3), 4
1.436
0.103
16.362(1)
14.3991(9)
14.4267(9)
90
91.266(1)
90
3398.1(4), 8
1.358
0.092
9.140(3)
9.5051(11)
13.026(5)
81.99(6)
81.50(7)
63.11(5)
994.9(5), 2
1.313
24.893(5)
9.1295(18)
16.445(3)
90
115.623(3)
90
3369.8(11), 8
1.310
0.088
β/Њ
γ/Њ
U/Å3, Z
Dc/Mg mϪ3
µ/mmϪ1
0.740
Reflections collected
Independent reflections (Rint
R1, wR2 [I > 2σ(I )]
7616
2161 (0.1445)
0.0687, 0.1269
7218
2432 (0.0139)
0.0318, 0.0840
8241
3520 (0.0732)
0.0556, 0.1349
6961
2386 (0.0237)
0.0362, 0.0912
)
(16.86)%. δH (CDCl3): 10.25 (br s, 1H, NH), 10.16 (br s, 1H,
NH), 8.54 (m, 2H, C5H4N), 8.29 (m, 2H, C5H4N), 7.87 (m, 2H,
C5H4N), 7.73 (d, 1H, J = 1.5, C6H3), 7.69 (d, 1H, J = 8.5, C6H3),
7.43 (m, 2H, C5H4N), 7.09 (dd, 1H, J = 1.5 and 8.5 Hz, C6H3),
2.38 (s, 3H, CH3). Selected IR bands (cmϪ1): 3302s (νNH), 1690s,
1660s (νCO). EI MS: m/z 332, Mϩ.
crystals of 4 were grown by evaporation of a dmf solution of
the complex.
3: Found (calc for C18H12CuN4O2): C 56.91 (56.56), H 2.75
(3.18), N 14.68 (14.75)%. Selected IR bands (cmϪ1): 1636s (νCO).
FABϩ MS: m/z 380, Mϩ.
4: Found (calc. for C19H14CuN4O2): C 56.57 (57.94), H 2.89
(3.58), N 13.94 (14.22)%. Selected IR bands (cmϪ1): 1641s (νCO).
FABϩ MS: m/z 394, Mϩ.
[Cu(py)4][ClO4]2
Copper() perchlorate hexahydrate (10.0 g, 27.0 mmol) was
dissolved in pyridine (100 cm3), giving a purple solution with
the evolution of some heat. The solution was evaporated to dry-
ness and the crude product recrystallised from nitromethane as
purple needles. Yield 12.50 g (80%). Found (calc. for C20H20-
Cl2CuN4O8): C 41.80 (41.50), H 2.86 (3.48), N 9.60 (9.68)%.
Reaction of H2L8 with copper(II) tetrakis(pyridine) perchlorate
Sodium hydride (57 mg, 2.4 mmol, 60% in mineral oil) was
washed with hexane (2 × 10 cm3) under dinitrogen and then
H2L8 (1.2 mmol) added as a solid in one portion. Thf (15 cm3)
was added then a solution of copper() tetrakis(pyridine) per-
chlorate (70 mg, 1.2 mmol) in acetonitrile (40 cm3) via cannula,
instantly giving a green suspension. Stirring was continued for
18 h, after which time the dark green product was filtered off,
washed with acetonitrile (50 cm3) and air-dried (35% yield).
Crystals of [Cu(L8)(OH2)]2ؒ2H2O 5 suitable for X-ray crystal-
lographic analysis were grown by vapour diffusion of diethyl
ether into a dmf solution of the complex. FABϩ MS: m/z 764,
[Cu2(L8)2]ϩ. Selected IR bands (cmϪ1): 1608s (νCO).
[Cu3(L7)2(ꢀ2-OAc)2] 1
Under aerobic conditions, a solution of copper() acetate
monohydrate (0.94 g, 4.6 mmol) in methanol (15 cm3) was
added dropwise over 5 min to H2L7 (1.0 g, 3.1 mmol) in
hot chloroform (60 cm3), giving an olive green solution. The
reaction mixture was stirred overnight and the solution was
concentrated under reduced pressure to ca. 5 cm3. Addition of
diethyl ether (100 cm3) afforded an olive green solid. Yield
1.20 g (83%). Dark green needles of 1 suitable for X-ray analysis
were grown by vapour diffusion of diethyl ether into a chloro-
form solution of the complex. Found (calc. for C38H28Cu3N10O8ؒ
CHCl3): C 44.36 (44.08), H 2.46 (2.75), N 13.20 (13.18)%.
Selected IR bands (cmϪ1): 1630s (νCO). FABϩ MS: m/z 824,
[Cu3(L7)2]ϩ.
The reaction of H2L7 with copper() tetrakis(pyridine) per-
chlorate was conducted using a similar procedure to give as
product in 30% yield.
[Cu3(L7)2(py)2][ClO4]2 6
Found (calc. for C44H32Cl2Cu3N12O8.C5H5N): C 46.06 (46.65),
H 2.56 (2.96), N 15.11 (14.43)%. Selected IR bands (cmϪ1):
1619s (νCO), 1085m, 624m (νperchlorate). FABϩ MS: m/z 825
[Cu3(L7)2]ϩ, 320 [HL7]ϩ.
[Cu3(L6)2(ꢀ2-OAc)2] 2
A suspension of copper() acetate monohydrate (279 mg, 1.40
mmol) and H2L6 (300 mg, 0.93 mmol) in 1 : 1 v/v methanol–
chloroform (40 cm3) was heated at reflux for 2 h under aerobic
conditions. The solution was filtered and the filtrate taken to
dryness in vacuo to give a green solid. Yield 0.43 g (96%). Dark
green needles of 2 suitable for X-ray analysis were grown by
diffusion of diethyl ether vapour into an acetonitrile solution
of the complex. Found (calc. for C34H24Cu3N14O8): C 43.03
(43.11), H 2.39 (2.55), N 20.55 (20.77)%. Selected IR bands
(cmϪ1): 1629s (νCO). ES MS: m/z 829, [Cu3(L6)2]ϩ.
X-Ray crystallography
X-Ray diffraction studies on H2L8ؒH2O, H2L9, H2L12, 1ؒCHCl3ؒ
0.25H2O, 2ؒ1.5CH3CNؒH2O, 4, 5ؒ2H2O and 7ؒC2H5OHؒ2H2O
were performed using a Bruker SMART-CCD diffractometer
with graphite-monochromated Mo-Kα radiation, H2L10ؒH2O
was studied on a Rigaku Mercury diffractometer with Cu-Kα
radiation. Details of data collections and structural refinements
for ligands and metal complexes are collected in Tables 1 and 2,
respectively. All data were corrected for Lorentz polarisation
and absorption. The structures were solved by direct methods,
except for 2 and 7 non-hydrogen atoms were refined with
anisotropic displacement parameters, hydrogen atoms bound to
carbon were idealised. Amine protons were located using ∆F
maps and allowed to refine subject to a distance constraint,
water bound O–H protons were fixed (O–H 0.98 Å). In H2L12
and 4 the methyl substituent was disordered over two sites – the
[CuL11] 3 and [CuL12] 4
These were prepared by reaction of stoichiometric amounts
of copper() acetate monohydrate with H2L11,12 in methanol
at room temperature and recrystallised from acetonitrile as
golden–brown solids in 90–95% isolated yields. X-Ray quality
D a l t o n T r a n s . , 2 0 0 4 , 8 6 2 – 8 7 1
864

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Table 2 Details of data collections and structural refinements for 1ؒCHCl3, 2ؒ1.5CH3CNؒ0.5H2O, 4, 5ؒ2H2O and 7ؒC2H5OHؒ2H2O
1ؒCHCl30.25H20
2ؒ1.5CH3CNؒ0.5H2O
4
5ؒ2H2O
7ؒC2H5OHؒ2H2O
Empirical formula
M
C39H29.5Cl3Cu3N10O8.25
1067.2
C37H29.5Cu3N15.5O8.5
1017.88
C19H14CuN4O2
393.88
C34H30Cu2N10O8
833.76
C70H54Cu4N20O13
1637.49
T /K
λ/Å
Crystal system
Space group
293(2)
0.71073
Monoclinic
P21/n
125(2)
0.71073
125(2)
125(2)
125(2)
0.71073
0.71073
Orthorhombic
Pbcn
0.71073
Monoclinic
C2/c
Triclinic
Triclinic
¯
¯
P1
P1
a/Å
b/Å
c/Å
α/Њ
12.7236 (10)
14.9250(11)
22.1202(17)
90
97.379(2)
90
4165.8(6),4
1.702
1.777
10.9969(14)
18.980(3)
20.125(3)
101.014(2)
101.375(2)
99.055(2)
3958.4(9), 4
1.708
18.777(5)
12.749(3)
6.8213(17)
90
90
90
1633.0(7), 4
1.602
1.360
18.377(4)
8.9474(15)
19.422(3)
90
93.094(5)
90
3188.8(10), 4
1.737
1.408
12.741(3)
13.793(3)
20.585(4)
93.395(4)
92.971(4)
107.026(3)
3443.8(13), 2
1.579
β/Њ
γ/Њ
U/Å3, Z
Dc/Mg mϪ3
µ/mmϪ1
1.674
1.299
Reflections collected
Independent reflections (Rint
R1, wR2 [I > 2σ(I )]
17896
5950 (0.0556)
0.0356, 0.0688
20088
11343 (0.1455)
0.0952, 0.1685
6185
1173 (0.0796)
0.0365, 0.0760
7579
2249 (0.0595)
0.0496, 0.1248
16877
9502 (0.0543)
0.1286, 0.3225
)
corresponding part weight phenyl protons were not included
in the refinement. In 5 the coordinated and solvate water mole-
cules were refined as rigid bodies with the hydrogen atoms on
the coordinated water being refined isotropically and the
solvate hydrogen atoms refined in riding geometries; these
protons are fairly poorly behaved. The refinement of the
structures of 2 and 7 proved difficult, several different crystals
were investigated in both cases but the poor quality of the data
obtained did not allow a full refinement but is adequate in pro-
viding gross structure/connectivity; in both cases anisotropic
refinement of the light atoms did not prove feasible. Structural
refinements were by the full-matrix least-squares method on F 2
using the program SHELXTL.18
CCDC reference numbers 227112–227120.
See http://www.rsc.org/suppdata/dt/b3/b316519a/ for crystal-
lographic data in CIF or other electronic format.
Results and discussion
Ligand syntheses
We have synthesised seven new polydentate pyridine dicarb-
oxamide ligands H2L1–7 from pyridine-2,6-dicarboxylic acid
(H2dipic), pyridine-2,6-dicarbonyl dichloride or 2,6-diamino-
pyridine (Fig. 1). Compounds H2L1–3 were generated from
reactions of H2dipic with 2-aminothiazole, 2-amino-5-chloro-
pyridine or 2-amino-4-methylpyridine, respectively, in the
presence of triphenylphosphite. When 2-aminopyrimidine
was used under analogous conditions the diester O,OЈ-
diphenylpyridine-2,6-dicarboxylate was isolated rather than the
desired dicarboxamide H2L4, phenoxide presumably originating
from triphenylphosphite. To overcome this problem H2L4
was synthesised from pyridine-2,6-dicarbonyl dichloride and
2-aminopyrimidine with triethylamine as hydrogen chloride
acceptor; similarly, H2L5 was prepared using 2-(aminomethyl)-
naphthalene. Treatment of 2,6-diaminopyridine with pyrazine-
2-carboxylic acid or 2-picolinic acid (Hpic) gave H2L6 and
H2L7, respectively. Using literature procedures, H2L8 was
prepared by a triphenylphosphite promoted condensation
reaction between 2-aminopyridine and H2dipic, while H2L9 and
H2L10 were synthesised by reaction of 2-(aminomethyl)pyridine
or 2-(aminoethyl)pyridine respectively with pyridine-2,6-
dicarbonyl dichloride in the presence of triethylamine.12,15 The
condensation reaction between 1,2-phenylenediamine and Hpic
generates 1,2-bis(pyridine-2-carboxamido)benzene (H2L11);16,17
3,4-bis(pyridine-2-carboxamido)toluene (H2L12) was prepared
from 3,4-diaminotoluene by the same route (Fig. 2). 4-Nitro-
1,2-phenylenediamine did not react with Hpic, as the electron
withdrawing influence of the nitro group deactivates the amine
groups from undergoing condensation.
Fig. 1 Reaction of H2dipic or pyridine-2,6-dicarbonyl dichloride with:
(i) 2-aminopyridine, () 2-(aminomethyl)pyridine, (iii) 2-(aminoethyl)-
pyridine, (iv) 2-(aminomethyl)naphthalene, (v) 2-amino-4-methyl-
pyridine, (vi) 2-amino-5-chloropyridine, (vii) 2-aminothiazole, (viii)
2-aminopyrimidine, (ix) 2-pyrazinecarboxylic acid and (x) Hpic.
Fig. 2 Synthesis of H2L11 (upper) and H2L12 (lower).
Compound H2L7 is regioisomeric with N,NЈ-bis(2-pyridyl)-
pyridine-2,6-dicarboxamide (H2L8).19 The thiazole (H2L1),
pyrimidine (H2L4) and pyrazine (H2L6) derivatised molecules
D a l t o n T r a n s . , 2 0 0 4 , 8 6 2 – 8 7 1
865

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Table 3 Selected bond lengths (Å) and angles (Њ) for H2L8ؒH2O, H2L9
and H2L10ؒH2O (esds in parentheses)
Table 4 Hydrogen bonding distances (Å) and angles (Њ) for H2L8ؒH2O,
H2L9 and H2L10ؒH2O (esds in parentheses).a The hydrogen bond donor
atoms are denoted as D and the acceptors as A
H2L8ؒH2O
H ؒ ؒ ؒ A
D ؒ ؒ ؒ A
D–H ؒ ؒ ؒ A
C(2)–N(7)
N(7)–C(8)
C(8)–O(8)
1.426(10)
1.361(9)
1.252(7)
C(15)–O(15)
C(15)–N(16)
N(16)–C(17)
1.231(8)
1.327(10)
1.441(10)
H2L8ؒH2O
N(7)–H(7N) ؒ ؒ ؒ N(10)
N(7)–H(7N) ؒ ؒ ؒ O(30)
N(16)–H(16N) ؒ ؒ ؒ N(10)
N(16)–H(16N) ؒ ؒ ؒ O(30)
O(30)–H(30A) ؒ ؒ ؒ N(1)
O(30)–H(30B) ؒ ؒ ؒ N(18)
2.19(1)
2.47(1)
2.19(1)
2.50(1)
2.35(1)
2.02(1)
2.661(10)
3.262(8)
2.675(10)
3.249(9)
2.922(9)
2.947(8)
107.8(1)
138.1(1)
109.2(1)
132.6(1)
116.7(1)
158.1(1)
C(8)–N(7)–C(2)
O(8)–C(8)–C(9)
129.3(6)
121.1(7)
O(8)–C(8)–N(7)
N(7)–C(8)–C(9)
123.4(7)
116.5(7)
H2L9
C(7)–N(8)
C(9)–O(9)
C(16)–N(17)
1.4518(18)
1.2294(18)
1.3289(18)
N(8)–C(9)
C(16)–O(16)
N(17)–C(18)
1.3356(19)
1.2316(17)
1.446419)
H2L9
N(8)–H(8N) ؒ ؒ ؒ N(11)
N(17)–H(17N) ؒ ؒ ؒ N(11)
N(8)–H(8N) ؒ ؒ ؒ N(20#)
N(17)–H(17N) ؒ ؒ ؒ N(20#)
2.294(15)
2.271(16)
2.450(8)
2.210(9)
2.718(2)
2.690(2)
3.3342(17)
3.0742(18)
105.1(10)
104.6(11)
150.0(12)
146.4(13)
C(9)–N(8)–C(7)
N(8)–C(9)–C(10)
121.81(12)
115.16(12)
O(9)–C(9)–N(8)
O(9)–C(9)–C(10)
124.10(14)
120.69(14)
H2L10ؒH2O
C(8)–N(9)
C(10)–O(10)
C(17)–N(18)
1.452(3)
1.238(3)
1.338(3)
N(9)–C(10)
C(17)–O(17)
N(18)–C(19)
1.332(3)
1.226(3)
1.456(3)
H2L10ؒH2O
N(9)–H(9N) ؒ ؒ ؒ O(30)
N(18)–H(18N) ؒ ؒ ؒ O(30)
O(30)–H(30B) ؒ ؒ ؒ N(22)
O(30)–H(30A) ؒ ؒ ؒ N(1#)
a For H2L9 and H2L10ؒH2O ‘#’ denotes atom from second molecule in
the dimer.
2.082(15)
2.328(10)
1.969(15)
2.166(14)
2.961(3)
3.275(4)
2.902(3)
3.120(3)
148(2)
163(2)
159(3)
164(4)
C(10)–N(9)–C(8)
O(10)–C(10)–C(11)
121.90(19)
120.0(2)
O(10)–C(10)–N(9)
N(9)–C(10)–C(11)
123.2(2)
116.86(18)
add further chemical and structural diversity to the existing
pyridine dicarboxamide ligands in the literature.
The new compounds H2L1–7 12
are air- and moisture-stable
,
between H(30A) and H(30B) of the water of crystallisation
with N(1) and N(18) of the terminal pyridyl rings. In contrast
to H2L8ؒH2O, H2L9 crystallises as dimer pairs of molecules
crosslinked by four-membered hydrogen bonded rings involving
pyridyl nitrogen atoms and NH groups (Fig. 4). H(8N) and
H(17N) form intramolecular hydrogen bonds to N(11) and to
N(20A) from the second molecule in the dimer, with N(20)
solids, with slight to moderate solubilities in chloroform and
thf but they are insoluble in methanol and diethyl ether. Micro-
analysis revealed that several of the compounds crystallised as
hydrates upon recrystallisation from wet solvents. In their IR
spectra, νNH and νCO bands appeared between 3351–3242 cmϪ1
and 1718–1656 cmϪ1 respectively, in their 1H NMR spectra the
NH resonance was a broad singlet in the region δ 10–12, with
the exception of H2L5, for which the resonance was obscured by
the aromatic protons.
X-Ray crystallographic analysis
Despite using several crystallisation techniques it proved
impossible to grow X-ray quality crystals of H2L1–7. For
comparative purposes we obtained the crystal structures of
N,NЈ-bis(2-pyridyl)pyridine-2,6-dicarboxamide monohydrate
(H2L8ؒH2O), N,NЈ-bis[2-(2-pyridyl)methyl]pyridine-2,6-dicarb-
oxamide (H2L9) and N,NЈ-bis[2-(2-pyridyl)ethyl]pyridine-2,6-
dicarboxamide monohydrate (H2L10ؒH2O), for which crystallo-
graphic analyses have hitherto not been reported. Selected bond
lengths and angles are given in Table 3, hydrogen bonding
parameters appear in Table 4.
The molecular framework of H2L8 is essentially planar, with
the C(8)–O(8) and C(15)–O(15) vectors oriented outwards from
the central pocket in which the water molecule resides (Fig. 3).
An extensive array of hydrogen bonding interactions exists
Fig. 4 (a) Molecular structure of H2L9and (b) hydrogen-bonded dimer
Fig. 3 Molecular structure of H2L8ؒH2O.
D a l t o n T r a n s . , 2 0 0 4 , 8 6 2 – 8 7 1
formation in H2L9.
866

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Table 5 Selected bond lengths (Å) and angles (Њ) for H2L12 (esds in
hydrogen bonding reciprocally with the amide protons of the
second molecule N(1) and N(1A) are uninvolved in interactions
with amide protons either within the dimer or to adjacent
molecules.
parentheses)
C(7)–O(7)
C(7)–N(8)
1.2254(19) O(16)–C(16)
1.352(2) N(15)–C(16)
1.2313(19)
1.347(2)
H2L10ؒH2O combines structural motifs from H2L8ؒH2O and
H2L9 (Fig. 5). Two H2L10 molecules adopt a head-to-tail
arrangement with H(9N), H(18N) and N(22) hydrogen bonding
with one water molecule while N(1) hydrogen bonds to a second
water molecule [H ؒ ؒ ؒ A 1.969(15)–2.328(10) Å, D ؒ ؒ ؒ A
2.902(3)–3.275(4) Å]. The arms of H2L10 hydrogen bond
independently of one another, the dimeric structure being
maintained solely by the water molecules, as there are no other
intra-dimer interactions.
C(16)–N(15)–C(10) 125.41(14) C(7)–N(8)–C(9)
125.85(14)
O(7)–C(7)–N(8)
N(8)–C(7)–C(2)
O(7)–C(7)–C(2)
124.87(15) O(16)–C(16)–N(15) 123.12(15)
113.78(14) N(15)–C(16)–C(17) 115.64(14)
121.34(15) O(16)–C(16)–C(17)
121.23(14)
Reactions of pyridine dicarboxamide ligands with copper(II)
The precise structure adopted by copper() complexes of pyr-
idine dicarboxamide ligands is dependent upon the flexibility of
the ligand. Kurosaki et al. demonstrated that ligands similar
to H2L10 containing histidine substituents rather than pyridyl
groups reacted with copper() acetate monohydrate to form
mononuclear complexes;3 X-ray crystallographic analysis con-
firmed deprotonation of the amide groups and chelation of the
copper() centre by the five nitrogen atoms of the dianion.
N,NЈ-bis(phenyl)pyridine-2,6-dicarboxamide (H2LPh) in its
dianionic form (LPh)2Ϫ reacted with copper() acetate mono-
hydrate to give [Cu2(LPh)2(µ-OH)]Ϫ, in which (LPh)2Ϫ is tri-
dentate.21 As the ligand arms elongate by one methylene unit
dimeric10 [CuL9]2 becomes monomeric [CuL10] in which (L10)2Ϫ
is pentadentate with a distorted square pyramidal arrangement
at copper().12 Also, H[CuBr2(S-PEPDAH)], where S-PEP-
DAH2
=
N,NЈ-bis[S-1-(2-pyridyl)ethyl]pyridine-2,6-dicarb-
oxamide, in which the anion coordinates through the nitrogen
atoms of a terminal pyridine, a deprotonated carboxamide
and the central pyridine, undergoes reversible conversion to a
dinuclear chiral helicate [Cu(S-PEPDA)]2 when treated with tri-
ethylamine.22 N,NЈ-Bis(8-quinolyl)pyridine-2,6-dicarboxamide
selectively extracted copper() ions into chloroform from an
aqueous phase containing several divalent ions, suggesting a
potential new application for pyridine dicarboxamide ligands.23
Copper() complexes with pyridine dicarboxamide ligands
have been prepared using copper() acetate monohydrate and
copper() tetrakis(pyridine) perchlorate as precursors. As
expected, there are substantial differences in the products
obtained using these two complexes.
Fig. 5 Molecular structure of H2L10ؒH2O (C–H bonds omitted for
clarity).
In H2L8–10 the carbonyl groups do not hydrogen bond with
amine protons or water molecules. The C–Namide and C᎐O
᎐
distances [1.317(12)–1.376(10), 1.226(3)–1.247(8) Å] are in
accord with 1,2-bis(pyridine-2-carboxamido)benzene and 1,2-
bis(pyridine-2-carboxamido)cyclohexane [1.321(2)–1.343(3),
1.222(3)–1.229(3) Å].1 X-Ray crystallography has revealed that
N,NЈ-dimethylpyridine-2,6-dicarboxamide and pyridine-2,6-
dicarboxamide form hydrogen bonding networks in the solid
state both intra- and intermolecularly, which high-temperature
1H NMR spectroscopy shows are retained in dmso solution up
to 150 ЊC.20 Notably, despite their crystallisation from strongly
hydrogen bonding media (water, dmso, ethanol) no solvent
inclusion occurred in either structure, thus the hydrate form-
ation in the structures of H2L8, H2L10 and several of the ligands
H2L1–7 is unexpected.
Reaction of copper(II) acetate monohydrate with H2L6,7
Copper() acetate monohydrate reacted with H2L7 or H2L6
(molar ratio 3 : 2) in methanol-chloroform at room temperature
to give trimetallic species [Cu3(L7)2(µ2-OAc)2] 1 and [Cu3(L6)2-
(µ2-OAc)2] 2, respectively, which were characterized by X-ray
crystallography.
X-Ray crystallography of 1ؒCHCl3ؒ0.25H2O reveals that two
H2L7 ligands are doubly deprotonated at the amidic nitrogens
and span three copper atoms (Fig. 7, Table 6). The internal
copper centre Cu(3) is bound by the nitrogen atoms N(10) and
N(40) from the central pyridyl rings of each (L7)2Ϫ anion, while
For H2L12 (Fig. 6, Table 5) the pyridyl ring planes are twisted
in opposite senses about the N–Ctolyl bonds relative to the
toluene moiety, structurally analogous with H2L11.1
Fig. 7 Molecular structure of 1ؒCHCl3ؒ0.25H2O (C–H bonds and
chloroform solvate molecule omitted for clarity).
Fig. 6 Molecular structure of H2L12.
D a l t o n T r a n s . , 2 0 0 4 , 8 6 2 – 8 7 1
867

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Table 6 Selected bond lengths (Å) for 1
Cu(1)–N(8)
Cu(1)–N(31)
Cu(1)–N(1)
Cu(2)–N(18)
Cu(2)–N(15)
Cu(3)–O(62)
Cu(3)–N(40)
1.966(3)
1.985(3)
2.110(3)
1.985(3)
2.062(3)
1.976(3)
2.007(3)
Cu(1)–N(38)
Cu(1)–O(61)
Cu(2)–N(45)
Cu(2)–O(64)
Cu(2)–N(48)
Cu(3)–O(63)
Cu(3)–N(10)
2.065(3)
2.016(3)
1.963(3)
2.026(3)
2.120(3)
1.938(3)
1.989(3)
Cu(1) and Cu(2) are chelated by the carboxamide and pyridyl
nitrogen atoms of each ligand arm to give two CuC2N2 rings
per terminal metal centre. Two µ2-acetato groups bridging
adjacent copper centres [Cu(1) ؒ ؒ ؒ Cu(3) and Cu(3) ؒ ؒ ؒ Cu(2)]
complete the molecular structure. The (L7)2Ϫ ligands have what
can loosely be described as a helicity, which is best illustrated by
a consideration of N–Cu–Cu–N torsion angles; N(1)–Cu(1)–
Cu(3)–N(10) and N(10)–Cu(3)–Cu(2)–N(18) are ca. Ϫ60Њ and
ϩ70Њ, while N(31)–Cu(1)–Cu(3)–N(40) and N(40)–Cu(3)–
Cu(2)–N(48) are ca. ϩ70Њ and Ϫ60Њ, respectively. This twisting
of the ligand framework about the amide groups curtails
Fig. 8 Molecular structure of 2ؒ1.5CH3CNؒ0.5H2O (C–H bonds and
solvate molecules omitted for clarity).
Spectroscopic and microanalytical data are in accord with
the crystallographically determined structures of 1 and 2, FABϩ
MS showed prominent peaks due to [Cu3L2]ϩ. One interesting
difference is found upon examination of the crystals under a
microscope; crystals of 1 are green whereas those of 2 appear
red, even though the bulk material is green.
electron delocalisation through the entire π-system of (L7)2Ϫ
.
Ligand helicity is also observed in [CuL9]2,10 the methylene
groups of (L9)2Ϫ allowing greater conformational flexibility of
the ligand backbone but with no potential for electron
delocalisation.
Cu(1) and Cu(2) possess distorted trigonal bipyramidal
[N4O] donor sets, with N(8) and N(31) for Cu(1) [N(18) and
N(45) for Cu(2)] occupying the axial sites, Cu(1) and Cu(2)
lying 0.08 and Ϫ0.07 Å, respectively, out of the [N2O] planes.
The cis angles within the [N2O] equatorial planes span the range
108–136Њ, the cis angles between axial and equatorial atoms are
from 80–100.0Њ. Cu(3) has approximately square planar cis-
[N2O2] geometry. The two CuC2N2 chelates formed by each
(L7)2Ϫ ligand at Cu(1) and Cu(2) exhibit an unexpected
asymmetry. Examining the (L7)2Ϫ ligand labelled N(1)–C(22),
the Cu(1)–N(8) and Cu(2)–N(15) (Cu–Namide) distances are
1.965(11) and 2.065(9) Å, respectively, while Cu(1)–N(1) and
Cu(2)–N(18) (Cu–Npyridyl) are 2.103(11) and 2.000(9) Å, i.e.
within each chelate ring there is one long and one short Cu–N
distance, the order reversing from one half of (L7)2Ϫ to the
other. The (L7)2Ϫ ligand labelled N(31)–C(52) behaves similarly.
Inspecting the Cu–Oacetate distances, Cu(3)–O(62) and Cu(3)–
O(63) [1.941(7), 1.958(7) Å] are ca. 0.07 Å shorter than Cu(1)–
O(61) or Cu(2)–O(62) [2.029(9), 2.036(8) Å]. The tricopper()
axis is obtuse [164.36(6)Њ] with intercopper distances
[Cu(1) ؒ ؒ ؒ Cu(3) 2.953(2), Cu(3) ؒ ؒ ؒ Cu(2) 2.900(2) Å] longer
than found in [Cu3(dpa)4Cl2]ؒH2O [2.471(1) Å, dpaH = 2,2Ј-
dipyridylamine]24 or [Cu2(OAc)4]ؒ2H2O (2.64 Å). There are
no significant intermolecular contacts within the structure.
Complex 2ؒ1.5CH3CNؒ0.5H2O is isostructural with 1 (Fig. 8,
Table 7). There are two independent molecules of 2 in the unit
cell, the lengths and angles for the second molecule are given in
square brackets in Table 7. Cu(1), Cu(2) and Cu(3) in molecule
I are designated as Cu(4), Cu(5) and Cu(6), respectively, in
molecule II, for the remaining ligand atoms, the atomic descrip-
tors are sixty higher than in molecule I, e.g. N(8) in molecule I is
N(68) in molecule II. Poor crystal quality precluded full refine-
ment of the structure for 2 and so no detailed discussion is
possible, though it does appear that the Cu(1)–O(23) and
Electrochemistry of 1,2
Complexes 1 and 2 have largely similar cyclic voltammetric
responses, as they have the same arrangement of donor atoms
and vary only by four atoms per molecule. Both display
chemically irreversible reductions near Ϫ1 V vs Fcϩ/0, close to
the expected potential for the copper(/) process. At lower
temperatures (Ϫ40 ЊC), at scan rates up to 40 V sϪ1 at a
platinum microelectrode this process remained irreversible.
Chronoamperometric studies (voltage stepped from 0.0 to
Ϫ1.2 V) confirmed that three electrons were involved, eqn. (1):
[Cu()–Cu()–Cu()] ϩ 3e
[Cu()–Cu()–Cu()]3Ϫ (1)
However, it should be noted that the assignment of this
process to the reduction of all three copper atoms makes the
assumption that there are no following reactions which could
lead to products which are themselves reducible at this poten-
tial. After the copper reduction of 1 and 2 at Ϫ1 V, two new
anodic peaks appear near Ϫ0.7 and Ϫ0.2 V. These peaks appear
to be due to soluble species as their peak heights increase with
the square root of scan rate, rather than linearly, as would be
expected for a wave due to an adsorbed species. Interestingly,
when using the superior solvent, dmf, the copper reduction of 2
was suppressed almost completely, so that the apparent number
of electrons was approximately one. We suggest that the com-
plex may be strongly adsorbed on the electrode, perhaps via
the pyrazine nitrogen atom, and that only one copper centre is
electrochemically accessible. Other electrode surfaces (gold and
platinum) exhibited similar behaviour with this solvent.
Other redox processes could be assigned as ligand-based
after comparison with the free ligand cyclic voltammetric data
(Table 8), which on account of very poor solubility could only
be measured for 2 in dmf and dichloromethane. The com-
plexes also showed an irreversible, approximately two-electron
oxidation process at more negative potentials than the ligand
Cu(2)–O(53) lengths are marginally shorter than
1 the
Cu ؒ ؒ ؒ Cu distances and tricopper axis being very similar to 1.
Table 7 Selected bond lengths (Å) for 2ؒ1.5CH3CNؒ0.5H2O (values for second independent molecule given in square brackets, esds in parentheses)
Cu(1)–N(8)
Cu(1)–N(31)
Cu(1)–N(1)
Cu(2)–N(18)
Cu(2)–N(15)
Cu(3)–O(24)
Cu(3)–N(10)
1.972(10) [1.965(10)]
2.032(11) [1.982(10)]
2.122(11) [2.165(10)]
2.015(10) [2.000(10)]
2.103(10) [2.037(10)]
1.947(9) [1.948(8)]
Cu(1)–O(23)
Cu(1)–N(38)
Cu(2)–N(45)
Cu(2)–O(53)
Cu(2)–N(48)
Cu(3)–O(54)
Cu(3)–N(40)
1.978(9) [1.991(8)]
2.107(11) [2.046(10)]
1.993(9) [1.986(10)]
2.024(8) [1.987(8)]
2.112(10) [2.181(9)]
1.954(8) [1.971(8)]
1.984(10) [2.000(10)]
1.965(10) [1.994(10)]
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Table 8 Cyclic voltammetric data (100 mV sϪ1) for [Cu3(L7)2(µ2-OAc)2] 1, [Cu3(L6)2(µ2-OAc)2] 2 and ligands (1 mM) in 0.1 M [Bu4N][PF6]
(V vs. Fcϩ/0) using a glassy carbon electrode (area 0.071 cm2)
Compound (solvent)
LϪ/2Ϫ
L0/Ϫ
Cu(/)
Cu(/)
Lϩ/0
1 (dmf )
Ϫ2.865b
Ϫ2.81a
Ϫ1.200b
Ϫ1.148b
Ϫ1.144b
Ϫ1.019b
–
–
–
–
1 (MeCN)
2 (dmf )f
2 (MeCN)
Ϫ2.623 (164)b
Ϫ2.373b, f
Ϫ2.619a
ϩ0.954c
ϩ0.840c
ϩ1.188c
Ϫ2.373a, d
Ϫ2.353 (143)
Ϫ2.642 (114)
ϩ1.607c
a Diffusion coefficients determined to be 1.60 and 1.30 0.2 × 10Ϫ5 cm2 sϪ1 for 1 and 2, respectively. b Cathodic peak only. c Anodic peak only.
d Compound 2 was much less soluble in acetonitrile, and the cyclic voltammograms were run for this complex at half the concentration (0.5 mM).
e Single, merged two-electron process per ligand in the complex. f Ligand has irreversible one-electron reductions recorded at Ϫ2.491 and Ϫ2.152 V in
dmf. Oxidation at ϩ1.397 V (irreversible; one electron process) in dichloromethane.
Table 9 Selected bond lengths (Å) and angles (Њ) for 4 (esds in
parentheses)
oxidation, tentatively assigned to the oxidation of the outer
copper centres, (eqn. (2));
Cu(1)–N(1)
C(7)–O(7)
N(8)–C(9)
2.023(3)
1.238(4)
1.395(5)
Cu(1)–N(8)
C(7)–N(8)
1.930(3)
1.343(5)
[Cu()–Cu()–Cu()] Ϫ 2e
[Cu()–Cu()–Cu()]2ϩ (2)
copper in such amide environments are known to be oxidised
relatively more easily.25
N(8)#1–Cu(1)–N(8)
N(1)–Cu(1)–N(1)#1
N(8)#1–Cu(1)–N(1)
N(8)–Cu(1)–N(1)
N(8)#1–Cu(1)–N(1)#1
N(8)–Cu(1)–N(1)#1
82.92(18)
111.59(19)
165.11(13)
82.89(14)
82.89(14)
165.11(13)
In order to see if the oxidation was due to the central copper
atom [Cu(pic)2]ؒ2H2O was used as a model. It was found that
[Cu(pic)2]ؒ2H2O is very insoluble in most solvents. It showed
very broad CV peaks in dmf, but these sharpened up consider-
ably in acetonitrile–1% v/v pyridine. The copper(/) reduction
appeared at Ϫ0.987 V. Chronoamperometry confirmed this to
be a one-electron reduction, and the diffusion coefficient
was found to be (2.3 0.2) × 10Ϫ5 cm2 sϪ1. If the potential was
extended significantly more cathodically than Ϫ1.2 V then the
copper() reduction became reversible: copper deposition
occurred as evidenced by a surface wave at Ϫ2.077 V and a
stripping peak at Ϫ0.509 V, followed by a broad adsorbate peak
near Ϫ0.1 V. Presumably the copper() complex decomposes in
the presence of pyridine. Cyclic voltammograms of free Hpic
in acetonitrile or dmf showed only a reduction at ca. Ϫ2.5 V
which was not observed in the complex. Interestingly, the
plating of copper was suppressed in the presence of dioxygen.
C(6)–N(1)–C(2)
C(6)–N(1)–Cu(1)
C(2)–N(1)–Cu(1)
118.7(3)
131.2(3)
110.1(3)
Magnetochemistry of 1
The magnetic properties of 1 were measured between 1.8 and
300 K. Zero field cooled (ZFC) data was obtained in a meas-
uring field of 200 Oe. Field cooled data was also collected,
however no significant difference was observed between that
and ZFC. The raw magnetisation data was converted to suscept-
ibility using the relationship χ = M/H and adjusted to give the
molar susceptibility using the mass of sample measured and the
molecular weight of 1. The ZFC molar susceptibility data thus
obtained and a best least squares fit model are shown in Fig. 12.
The observed data were fitted to the model in eqn. (3)
Fig. 9 Magnetic susceptibility data for 1. Circles represent data points,
line is best fit to model.
Reaction of copper(II) acetate monohydrate with H2L11,12
[CuL11] 316 and [CuL12] 4 were prepared in near-quantitative
yields by reacting copper() acetate monohydrate with one
equivalent of the respective ligands in methanol. IR spectro-
χm = TIP ϩ C/(T ϩ T c)
(3)
scopy confirmed deprotonation as νNH bands were absent while
the νCO bands (1636, 1641 cmϪ1) were 40–50 cmϪ1 lower than
in H2L11,12. X-Ray crystallography showed 4 to have crystallo-
graphic C2 symmetry, the methyl group having equal occupancy
over two sites (Fig. 10, Table 9). The coordination sphere of
Cu(1) comprises pyridyl and carboxamide nitrogen atoms
[N(1), N(1#) and N(8), N(8#), respectively] in a distorted square
planar arrangement, with Cu–Namide shorter than Cu–Npyridyl
[1.929(4), 1.929(3) Å vs. 2.020(4) Å]. N(1)–Cu(1)–N(1#)
[111.7(2)Њ] is the largest cis angle [Σcis angles) = 360.3Њ], the
trans N(1)–Cu(1)–N(8) vector being 164.96(15)Њ. Although
C(7)–N(8) [1.351(5) Å] is essentially unchanged from H2L12
[1.356(4), 1.347(4) Å] the planarity of 4 permits greater electron
delocalisation within the structure compared to the free ligand.
We reported the electrochemistry for 3 and 4 in a preliminary
communication,27 and the results are summarised here. Their
cyclic voltammetric behaviour in dmf were very similar. For 4
the oxidation waves at ϩ0.4 V were similar to those observed
for [Cu(ppO2)]28 {H2ppO2 = N,NЈ-bis[2-(2-pyridyl)methyl]pro-
where TIP is a temperature-independent component to account
for diamagnetism in both the sample and the sample holder, C
is the Curie constant and T c is the Weiss or Curie temperature
depending on its sense. The fit yielded the following values; TIP
= 0.00589, C = 0.42509 and T c = Ϫ0.0369 K (standard error:
0.0015097, correlation coefficient: 0.9988735). The data could
be fitted using more complex models, however the fittings
obtained were no better than the one given above, hence this
simple model was used.
As Fig. 9 demonstrates, 1 behaves as a simple paramagnet.
Although a Weiss/Curie temperature is obtained, its value is so
small that it is probably best treated with caution. These results
are consistent with those of Fenton26 who noted an extremely
weak antiferromagnetic interaction between adjacent copper
ions [Cu ؒ ؒ ؒ Cu 3.985 Å] in another tricopper() complex,
where direct bridging species capable of super-exchange were
not present.
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Table 10 Selected bond lengths (Å) for 5ؒ2H2O (esds in parentheses)
Table 11 Selected bond lengths (Å) for 7ؒC2H5OHؒ2H2O (esds in
parentheses)
Cu(1)–N(10)
Cu(1)–N(1)#1
Cu(1)–N(16)
Cu(1)–N(7)
Cu(1)–O(30)
1.930(4)
1.983(4)
2.023(4)
2.048(4)
2.278(2)
N(1)–C(2)
N(1)–C(6)
N(1)–Cu(1)#1
C(2)–C(3)
C(2)–N(7)
1.338(6)
1.338(6)
1.983(4)
1.329(6)
1.398(6)
Cu(1)–N(8)
Cu(1)–N(38)
Cu(1)–O(10)
Cu(1)–N(1)
Cu(1)–N(31)
Cu(2)–N(105)
Cu(2)–N(15)
Cu(2)–N(18)
Cu(2)–O(10)
Cu(2)–N(108)
1.933(11)
1.958(12)
2.019(10)
2.066(11)
2.217(12)
1.933(12)
1.942(12)
2.052(12)
2.068(10)
2.208(14)
Cu(3)–N(68)
Cu(3)–N(45)
Cu(3)–N(61)
Cu(3)–O(70)
Cu(3)–N(48)
Cu(4)–N(75)
Cu(4)–N(98)
Cu(4)–O(70)
Cu(4)–N(78)
Cu(4)–N(91)
1.928(11)
1.943(12)
2.049(11)
2.085(10)
2.219(12)
1.926(11)
1.953(12)
2.075(11)
2.088(12)
2.216(12)
(L8)2Ϫ ligand, a pyridyl nitrogen atom [N(1A)] from the other
(L8)2Ϫ ligand in the dimer, and a water molecule. Cu(1) is
displaced by 0.06 Å out of the N(16)–N(10)–N(7)–N(1A)
square basal plane with O(30) the apical ligand. Upon
deprotonation of H2L8 both of the terminal pyridyl rings of
(L8)2Ϫ twist about the Cpyridyl–Namide bonds, although only one is
used for complexation to the second copper centre in the dimer.
Despite the outward orientation of one pyridyl ring and the
amide oxygen atoms from Cu(1) there are no significant inter-
molecular contacts.
Deprotonation slightly shortens C(8)–O(8) with N(7)–C(8)
[1.351(5) Å] being approximately the mean value of (C–N)amide
in H2L8 [1.346 Å]. The trans N(10)–Cu(1)–N(1#) angle
[169.39(14)Њ] is slightly narrower than for Cu(1) and Cu(2) in 1
[178.6(4), 173.8(4)Њ], which have trigonal bipyramidal [N4O]
donor sets. For 5 the Cu–Npyridyl lengths [Cu(1)–N(10) 1.931(4),
Cu(1)–N(1#) 1.988 Å] are shorter than Cu–Namide [Cu(1)–N(16)
2.032(3), Cu(1)–N(7) 2.048(4) Å], cf. the Cu–Namide and Cu–
Npyridyl distances in 1 and 4. Within the CuN4 plane the cis and
trans N–Cu(1)–N angles are 79.13(14)–101.44(14)Њ and
157.32(14)–169.39(14)Њ, respectively, the cis N–Cu(1)–O(30)
angles are 86.51(13)–105.98(14)Њ. Hydrogen bonding inter-
actions within the structure exist between coordinated and
solvate water molecules [H(30B) ؒ ؒ ؒ O(31) 1.868 Å,
H(31A) ؒ ؒ ؒ O(30) 2.1 Å, H(30B) ؒ ؒ ؒ O(15) 1.786 Å], the
pendant pyridyl group and the carbonyl oxygen atoms do not
Fig. 10 Molecular structure of 4.
pylenediamine-1,3-dione} and are assigned as copper(/)
processes. This was confirmed by spectroelectrochemistry of 4
which showed that the electrogenerated copper() species with
λmax at 370 and 640 nm had a UV-visible spectrum comparable
to [Cu(ppO2)]ϩ.
Reaction of copper(II) tetrakis(pyridine) perchlorate with
pyridine dicarboxamide ligands
The dianions generated from H2L2, H2L3, H2L4, H2L7 and H2L8
using sodium hydride in thf reacted with copper() tetra-
kis(pyridine) perchlorate in thf–acetonitrile to give green com-
plexes soluble in polar organic solvents (pyridine, dmf,
acetonitrile). It was difficult to crystallise the copper() com-
plexes by any crystallisation method, moreover satisfactory
elemental analyses could not be obtained owing to contamin-
ation by unreacted ligand or products whose identities are as
yet uncertain. In most cases, FABϩ or ES MS showed peaks
corresponding to [Cu2L2]ϩ with the appropriate isotopomeric
distribution, indicating that some proportion of the isolated
material contained a complex with this unit.
participate, cf. the structures of H2L8–10
.
Whereas FABϩ or ES MS data for the copper() complexes
of the pyridine-2,6-dicarboxamide derived ligands were gener-
ally consistent with their formulation as bimetallic species, for
the (L7)2Ϫ complex we believe that the product is a trimetallic
species of composition [Cu3(L7)2(py)2][ClO4]2 6, i.e. with a
similar trinuclear axis as 1. FABϩ MS showed a prominent peak
at m/z 825 for [Cu3(L7)2]ϩ, with νCO at 1619 cmϪ1 in the IR
spectrum, cf. 1630 cmϪ1 for 1. Perchlorate bands (1085, 624
cmϪ1) were also apparent. The microanalytical data for 6 sug-
gested the co-crystallisation of a molecule of pyridine. (L7)2Ϫ
It was possible to crystallise [Cu(L8)(OH2)]2ؒ2H2O (5ؒ2H2O)
from dmf–diethyl ether (Fig. 11, Table 10). The coordination
sphere of the square pyramidal copper atom comprises
the nitrogen atoms of the central pyridine ring [N(10)] and
the deprotonated carboxamide groups [N(7), N(16)] of one
displays substantially different behaviour to the isomeric (L8)2Ϫ
,
due to the impossibility of chelation to one metal centre by the
central pyridyl ring and the deprotonated carboxamide groups.
Attempts to obtain X-ray quality crystals of 6 were unsuc-
cessful, however an interesting transformation occurred in
ethanol. Following addition of ethanol to the complex and
standing for two days (aerobic conditions) green crystals were
isolated from the orange residue. X-Ray crystallographic analy-
sis (although a relatively poor quality data collection and an
isotropic refinement of the light atoms) revealed the complex to
be a tetracopper() species [Cu4(L7)2(L7-O)2]ؒC2H5OHؒ2H2O
(7ؒC2H5OHؒ2H2O) (Fig. 12, Table 11). There are two (L7)2Ϫ and
two (L7–O)2Ϫ ligands in the molecule, in the latter oxidation of
the nitrogen atom of the central pyridyl ring generates an
N-oxide. Each copper atom is bound by the nitrogen atoms of
the terminal pyridyl rings and deprotonated carboxamide
group from one (L7)2Ϫ ligand and by the same donor atoms
plus the N-oxo atom from an (L7–O)2Ϫ ligand, giving a square
pyramidal [N4O] coordination sphere at the metal centre. Each
N-oxygen atom bridges two adjacent copper centres. The cen-
Fig. 11 Molecular structure of 5ؒ2H2O (C–H bonds omitted for
clarity).
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acetonitrile–thf with (L8)2Ϫ to give a binuclear complex 5 and a
trimetallic structure 6 for (L7)2Ϫ. The grid-like structure of the
tetracopper complex 7 in which copper centres are directly
linked by µ2-O units is expected to lead to extremely interesting
magnetic and electrochemical properties. If its preparation is
indeed reproducible in good yield then other ligands containing
2,6-diaminopyridine units may exhibit similar reactivity to give
oligometallic frameworks.
Acknowledgements
This work was supported by the Israel Science Foundation.
We are grateful to Dr Martin B. Smith (University of St
Andrews) for magnetic measurements, the EPSRC (P. B.) and
the BBSRC (S. L. J.) for financial support.
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Fig. 12 Molecular structure of 7ؒC2H5OHؒ2H2O (C–H bonds and
solvate molecules omitted for clarity).
tral pyridyl ring of (L7)2Ϫ is too distant from any copper centre
to enable it to bind, hence these ligands are tetradentate
whereas (L7–O)2Ϫ is pentadentate. While coordination of pyr-
idine N-oxide ligands to copper() is common, as far as we
are aware the complexation of the N-oxide of a 2,6-diamino-
pyridine derived ligand is unique.
Complex 7 has a [2 × 2] grid structure with the (L7)2Ϫ ligands
being face-on to one another and perpendicular to the (L7–O)2Ϫ
ligands, which are themselves face-to-face. The poor quality
of the data collection precludes detailed discussion of the
structure but it does appear to have the same overall motif as
the recently reported pyrazine relative.14
A recent communication highlighted an unusual reactivity
of a pyridine dicarboxamide complex of copper(). When
[Cu(dmppy)(en)] (H2dmppy
= N,NЈ-dimethylpyridine-2,6-
dicarboxamide, en = 1,2-diaminoethane) was added to a
solution of en in acetonitrile, crystalline [Cu(en)3][Cu(CN)3]
was isolated quantitatively in addition to H2dmppy and a
methylated imidazoline derived polymer.29 The heterolysis of
acetonitrile into CH3ϩ and CNϪ fragments was attributed to the
highly basic nature of the (dmppy)2Ϫ ligand. A similar chem-
istry may exist for selected copper() complexes of H2L1–10
,
however no evidence has been found for reactivity of the
complexes described here towards acetonitrile or any other
solvent under ambient conditions.
19 E. S. Nikitskaja, V. S. Usovskaia and M. V. Rubtsov, Zh. Obsch.
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23 K. Hiratami and K. Taguchi, Bull. Chem. Soc. Jpn., 1990, 63, 3331.
24 L. P. Wu, P. Field, J. Morrissey, C. Murphy, P. Nagle, B. Hathaway,
C. Simmons and P. Thornton, J. Chem. Soc., Dalton Trans., 1990,
3835.
Summary and conclusions
A series of pyridine dicarboxamide ligands was prepared from
reactions of H2dipic, pyridine-2,6-dicarbonyl dichloride or 2,6-
diaminopyridine with amine or carboxylic acid functionalised
heterocycles. H2L8–10 crystallised with extensive hydrogen
bonding networks involving pyridyl nitrogen atoms, NH groups
and in some cases water of crystallisation. The carbonyl oxygen
atoms play no part in the hydrogen bonding arrays.
The structure-directing properties of pyridine dicarboxamide
ligands can be exploited to prepare a range of topologies for
copper() complexes owing to their inability to encapsulate
one metal centre due to the requirement for four-membered
metallacycle formation. H2L6 and H2L7 reacted with copper()
acetate monohydrate to give [Cu3L2(µ2-OAc)2] 1, 2 which are
the first crystallographically characterised metal complexes
containing a dicarboxamide ligand derived from 2,6-diamino-
pyridine. Copper() tetrakis(pyridine) perchlorate reacted in
25 J. Hanss, A. Beckmann and H. J. Kruger, Eur. J. Inorg. Chem., 1999,
163.
26 N. A. Bailey, D. E. Fenton, Q. Y. He, N. Terry, W. Haase and
R. Werner, Inorg. Chim. Acta, 1995, 235, 273.
27 S. L. Jain, J. A. Crayston, D. T. Richens and J. D. Woollins, Inorg.
Chem. Commun., 2002, 5, 853.
28 T. Sakurai, J. I. Hongo, A. Nakahara and Y. Nakao, Inorg. Chim.
Acta, 1980, 46, 205.
29 D. S. Marlin, M. M. Olmstead and P. K. Mascharak, Angew. Chem.,
Int. Ed. Engl., 2001, 40, 4752.
D a l t o n T r a n s . , 2 0 0 4 , 8 6 2 – 8 7 1
871